1. Field of the Invention
The present invention is directed to apparatus for testing oilfield cased holes.
2. The Prior Art
In an oil or gas well, it is customary to measure formation pressure and to sample reservoir fluid. Such testing is normally done in open well holes prior to setting the casing, and then after the well is cased, the testing process is repeated using full production testing equipment. Several tools are available to sample reservoir fluids and/or measure reservoir pressure prior to completing a well. These tools are lowered into the well on an armored electrical cable prior to the setting of the protective steel casing in that well. These tools usually have a sealing test pad that can be pressed against the formation by electrically powered hydraulics. The test paid has a probe or snorkel to penetrate the impermeable filter cake produced by drilling mud solids. Electrically controlled hydraulic valving opens the test probe to various sampling chambers and pressure measuring instruments so that a record of pressure versus time, as well as a fluid sample, can be taken from a given zone. Modern tools can take several separate samples from differing depths in order to increase the operating efficiency. The pressure measurement is recorded by the tool during each test allowing pressure transient analysis to be performed. Estimates of reservoir permeability can be made while the tool is still in the well.
There are several disadvantages to running these tools in "open-hole" before the protective casing is set. For example, the cost of the rig during testing can be prohibitive, particularly in offshore use. In addition, the drilling process typically provides for the presence of a fluid column in the open well with a slightly higher hydrostatic pressure (controlled typically by drilling fluid density) than the reservoir rock fluid pressure to prevent entry of reservoir formation fluids into the wellbore. The "overbalance," as it is called, leads to frequent differential sticking of the test tools, which necessitates the expensive and difficult task of retrieval. Sticking by this type of tool in uncased wellbores is common because of the extensive sealing pad contact area, and the fact that the tool remains motionless during the test.
A suitable tool to test reservoir pressures and sample fluids after setting the casing is therefore very desirable. In addition to the potential cost reduction for new wells, a cased hole testing tool allows the possibility to re-evaluate existing wells and examine zones that may contain hydrocarbons but were originally not the main objective. It would therefore be advantageous to make these measurements on wireline after the casing is set, without the use of production test equipment, especially to reevaluate old wells.
The process of setting a casing involves cementing the pipe into the drilled well bore to establish a hydraulic seal by solid cement between the pipe and the formation wall to isolate zones. Cased hole formation test tools therefore must penetrate both the pipe and the cement sheath in order to establish hydraulic communication to the reservoir being tested.
Existing cased hole testing tools generally have a sealing test pad that can be pressed against the casing bore by electrically powered hydraulics. A shaped explosive charge centered within the test pad is then used to make communication with the test zone. While effective, this method usually limits the tool to one test per trip in the well, and resealing of the resulting jagged test hole has been a significant problem. Pressure is measured from the zone using either strain or quartz pressure gauges, and a single sample of fluid can be obtained. Following the tests, the tool must be brought to the surface and redressed before additional tests can be conducted.
Dave, U.S. Pat. No. 5,195,588, discloses an apparatus in which an axially slidable gunblock contains a shaped charge perforating subsystem and a plugging subsystem at two different axial positions. After the perforation/testing function is performed, the gunblock is mechanically repositioned so the sealing function can be performed. Aside from the inherent problem of achieving a satisfactory seal against the severe irregularities caused by shaped charge perforation, the reliability of such a tool can suffer due to the inherent mechanical complexity involved with accurately moving the tool from the sampling position to the sealing position. The weight penalty associated with the mechanical complexity limits the number of gun block assemblies that can be suspended by cable.